1. Field of the Invention
This invention relates to a physical quantity detection device for detecting a physical quantity through resistance variation with temperature compensation.
2. Description of the Prior Art
A physical quantity detection device for detecting a physical quantity through resistance variation with temperature compensation is disclosed in Japanese patent No. 2976487. FIG. 5 is a schematic circuit diagram of this prior art physical quantity detection device (pressure sensor). This physical quantity detection device has four operational amplifiers OP11 to OP14.
The operational amplifier OP11 compensates a temperature characteristic of sensitivity by supplying a constant current to a Wheatstone bridge including strain gages for detecting a pressure. The operational amplifier OP12 and the operational amplifier OP13 are used as voltage-followers to suppress an error due to directly drawing an output current from the bridge output. The operational amplifier OP14 amplifiers the pressure detection signal and shifts the zero point. This physical quantity detection device provides a physical quantity detection signal with accuracy over a wide temperature range.
However, in consideration of reduction in cast and in the area occupied by this circuit on an IC chip, the number of the operational amplifiers should be reduced.
Japanese patent No. 3-67211 (U.S. Pat. No. 4,576,052) discloses another prior art physical quantity detection device satisfying this requirement. FIG. 6 is a schematic circuit diagram of this prior art physical quantity detection device including two operational amplifiers. This prior art physical quantity detection device provides the same function as the physical quantity detection device shown in FIG. 5 with only two operational amplifiers. In the case that the physical quantity detection device is used as a pressure sensor in motor vehicles, it is generally required that the physical quantity detection device is driven by a single power supply of 5 V, so that the output voltage range is from 0.5 to 4.5 V. Moreover, it is further required that the output voltage increases with an increase in pressure. Thus, in consideration of these general requirements, the prior art physical quantity detection device shown in FIG. 6 has the following disadvantages.
First, in the physical quantity detection device (pressure sensor) shown in FIG. 6, the operational amplifier OP21 is used as a differential amplifier. Thus, the operational amplifier OP21 outputs 0 V when there is no pressure on the stain gages Raxe2x80x2 to Rdxe2x80x2, and the output voltage increases with increase in the pressure.
In the general operational amplifier, the output voltage is limited at high and low voltages in the output voltage range. That is, near the supply voltage, the output voltage is limited by the voltage (Vcc) of the power supply, and near the ground level, the output voltage is subjected to the low voltage limit such as transistor""s |Vce(sat)|≈0.2 V. Moreover, when a difference between two pressures is measured, this circuit cannot generate a negative voltage, so that the output cannot represent the negative pressure.
Second, in the above-mentioned structure, because of the single supply voltage, the output voltage range starts from 0.5 V. Thus, it is required to shift the zero point of the output signal. Accordingly, the operational amplifier OP22 at the last stage should be used as an inverting type operational amplifier having a summing input. However, in this structure, the output voltage of the last stage of operational amplifier OP22 decreases with increase in the output voltage at the previous stage of the operational amplifier OP21 when the pressure increases. That is, the output characteristic of the pressure sensor shown in FIG. 6 is as shown in FIG. 7. This is an inverted characteristic with respect to the required output characteristic for the pressure sensor.
The pressure sensor disclosed in Japanese patent application provisional publication No. 3-51733 (U.S. Pat. No. 5,042,307) clears this requirement. FIG. 8 is a schematic circuit diagram of this prior art pressure sensor(physical quantity detection device). This pressure sensor is developed to have operation which is the same as the prior art pressure sensor having four operational amplifiers shown in FIG. 5. In this pressure sensor shown in FIG. 8, a feedback resistor Rh has a temperature dependency for compensating a temperature characteristic of sensitivity. Moreover, its offset temperature characteristic can be adjusted also. However, if only offset is simply adjusted, because the feedback resistor Rh has temperature dependency, in order to cancel the temperature dependency, it is necessary to equalize TCRs (temperature coefficients of resistance) in the combined resistance of the resistors Ri, R27, and R29 and the combined resistance of the resistors Rj, R28, and R30 to that of the combined resistance of the resistors Rh and R26.
Moreover, in the actual offset adjustment and the actual offset temperature adjustment, the resistors R27, R29, R28, and R30 are adjusted adequately. However, trimming any of these resistors varies the offset temperature characteristic due to the presence of the temperature dependency in the feedback resistor Rh. That is, in this pressure sensor, accurate adjustment cannot be provided because the offset and offset temperature dependency cannot be adjusted independently.
There is a further problem in this pressure sensor. That is, the non-inverting input of the operational amplifier OP32 at the second stage is supplied with the output of the bridge, so that this operational amplifier OP32 operates with a reference potential including the pressure detection signal which is not amplified. Thus, adjusting the offset and offset temperature characteristic in the second stage by trimming any of the resistors R27, R28, R30 slightly exercises an influence on the pressure detection signal component. That is, this structure exercises an influence on the adjustment in the sensitivity. Thus, if the offset and the offset temperature characteristic are adjusted after adjustment of the sensitivity, the accuracy in the sensitivity will decrease because the adjustment in the sensitivity deviates slightly from the previous state. Inversely, if the offset in the bridge, the offset (voltage), and the offset temperature characteristic are adjusted before the sensitivity adjustment, the offset and offset temperature characteristic deviates from the adjusted values.
Accordingly, the physical quantity detection device such as the pressure sensor is required to output an accurate detection signal with temperature compensation using a single power supply and a low number of amplifiers even at a voltage near the ground potential and a negative potential, and the adjustment should be easy with accuracy.
The aim of the present invention is to provide a superior physical quantity detection device.
According to the present invention, a first aspect of the present invention provides a physical quantity detection device including: an operational amplifier; a first resistor connected between an inverting input of the operational amplifier and a first reference potential; a second resistor connected between the inverting input of the operational amplifier and a second reference potential, the first and second resistors having a first temperature coefficient of resistance; a feedback resistor being connected between the inverting input of the operational amplifier and an output of the operational amplifier and having a second temperature coefficient of resistance; and a reference voltage generation circuit generating a reference voltage supplied to a non-inverting input of the operational amplifier, at least one of the first and second resistors including a sensing element of which resistance varies on the basis of a physical quantity with a temperature coefficient of sensitivity, wherein a difference between the first temperature coefficient of resistance and the temperature coefficient of sensitivity is substantially equal to the second temperature coefficient of resistance.
According to the present invention, a second aspect of the present invention provides the physical quantity detection device based on the first aspect, wherein each of the first and second resistors and the feedback resistor includes a diffused resistor and a concentration of impurity of the feedback resistor is different from concentrations of impurity of the first and second resistors.
According to the present invention, a third aspect of the present invention provides the physical quantity detection device based on the second aspect, wherein the concentrations of impurity of the first and second resistors are from 0.4xc3x971019 cmxe2x88x923 to 8xc3x971019 cmxe2x88x923 and the concentration of impurity of the feedback resistor is from 1.6xc3x971017 cmxe2x88x923 to 7xc3x971017 cmxe2x88x923. Moreover, the concentrations of impurity of the first and second resistors may be from 0.8xc3x971019 cmxe2x88x923 to 4xc3x971019 cmxe2x88x923 and the concentration of impurity of the feedback resistor is from 2.5xc3x971017 cmxe2x88x923 to 5.5xc3x971017 cmxe2x88x923. Furthermore, the concentrations of impurity of the first and second resistors may be about 1xc3x971019 cmxe2x88x923, and the concentration of impurity of the feedback resistor is about 4xc3x971017 cmxe2x88x923.
According to the present invention, a fourth aspect of the present invention provides the physical quantity detection device based on the first aspect, wherein one of the first and second resistors includes the sensing element of which resistance varies on the basis of the physical quantity, and a resistance of the other of the first and second resistors does not vary with the physical quantity.
According to the present invention, a fifth aspect of the present invention provides the physical quantity detection device based on the first aspect, wherein the reference voltage generation circuit includes third and fourth resistors connected in series between the first and second reference potentials and generates a divided voltage as the reference voltage, and a temperature coefficient of resistance of the third resistor is substantially equal to a temperature coefficient of resistance of the fourth resistor.
According to the present invention, a sixth aspect of the present invention provides the physical quantity detection device based on the fifth aspect, wherein one of the third and fourth resistors has a trimming structure to trim the reference voltage toward an output voltage of the operational amplifier when the physical quantity is zero.
According to the present invention, a seventh aspect of the present invention provides the physical quantity detection device based on the first aspect, further including a resistor having a trimming structure and that is connected in parallel to the feedback resistor.
According to the present invention, an eighth aspect of the present invention provides the physical quantity detection device based on the first aspect, further including: a third resistor; another operational amplifier having an inverting input supplied with an output of the operational amplifier through the third resistor, a non-inverting input of the another operational amplifier being supplied with the reference voltage; and a fourth resistor disposed between an output terminal and inverting input of the another operational amplifier.
According to the present invention, a ninth aspect of the present invention provides the physical quantity detection device based on the eighth aspect, further including an offset trimming resistor between the first reference potential and the inverting input of the another operational amplifier.
According to the present invention, a tenth aspect of the present invention provides the physical quantity detection device based on the eighth aspect, further including an offset trimming resistor between the second reference potential and the inverting input of the another operational amplifier.
According to the present invention, an eleventh aspect of the present invention provides the physical quantity detection device based on the eighth aspect, further including fifth and sixth resistors connected between the first reference potential and the inverting input of the second operational amplifier and seventh and eighth resistors connected between the inverting input of the another operational amplifier and the second reference potential, wherein the sixth and seventh resistors have temperature dependencies of resistance.
According to the present invention, a twelfth aspect of the present invention provides the physical quantity detection device based on the eleventh aspect, wherein at least one of the fifth and eighth resistors has a trimming structure for compensating a temperature characteristic of offset of the output of the another operational amplifier.
According to the present invention, a thirteenth aspect of the present invention provides the physical quantity detection device wherein if it is assumed that a sensitivity of the sensing element at a reference temperature is S0, a resistance of the sensing element at the reference temperature is R0, and a resistance of the feedback resistor at the reference temperature is Rts0, then, it is represented that the sensitivity of the sensing element at a temperature t which is different from the reference temperature by T is S(T), the resistance of the sensing element at t is R(T), and the resistance of the feedback resistor at t is Rts(T), and S(T), R(T), and Rts(T) are further represented by: S(T)=S0xc2x7(1+xcex21xc2x7T+xcex22xc2x7T2), R(T)=R0xc2x7(1+xcex11xc2x7T+xcex12xc2x7T2), and Rts(T)=Rts0xc2x7(1+A1xc2x7T+A2xc2x7T2), where xcex11, xcex12, xcex21, xcex22, A1, and A2 are temperature coefficients, and wherein xcex11, xcex12, xcex21, xcex22, A1, and A2 are determined so as to establish both A1=xcex11xe2x88x92xcex21 and A2=xcex12xe2x88x92xcex22xe2x88x92xcex21xc2x7(xcex11xe2x88x92xcex21).
According to the present invention, a fourteenth aspect of the present invention provides the physical quantity detection device based on the first aspect, wherein the reference voltage is determined such that almost all of a current flowing through the first resistor flows into the second resistor.
According to the present invention, a fifteenth aspect of the present invention provides a physical quantity detection device including: an operational amplifier; a first resistor connected between an inverting input of the operational amplifier and a first reference potential; a second resistor connected between the inverting input of the operational amplifier and a second reference potential, the first and second resistors having a first temperature coefficient of resistance; a feedback resistor being connected between the inverting input of the operational amplifier and an output of the operational amplifier and having a second temperature coefficient of resistance; a reference voltage generation circuit generating a reference voltage supplied to a non-inverting input of the operational amplifier, at least one of the first and second resistors including a sensing element of which resistance varies on the basis of a physical quantity with a temperature coefficient of sensitivity, wherein the reference voltage generation circuit includes a third resistor and a fourth resistor connected in series between the first and second reference potentials and generates a divided voltage as the reference voltage, and a temperature coefficient of the third resistor is substantially equal to a temperature coefficient of the fourth resistor.
According to the present invention, a sixteenth aspect of the present invention provides a physical quantity detection device including: an operational amplifier; a first resistor connected between an inverting input of the operational amplifier and a first reference potential; a second resistor connected between the inverting input of the operational amplifier and a second reference potential, the first and second resistors having a first temperature coefficient of resistance; a feedback resistor being connected between the inverting input of the operational amplifier and an output of the operational amplifier and having a second temperature coefficient of resistance; a reference voltage generation circuit generating a reference voltage supplied to a non-inverting input of the operational amplifier, at least one of the first and second resistors including a sensing element of which resistance varies on the basis of a physical quantity with a temperature coefficient of sensitivity, a third resistor; another operational amplifier, an inverting input of the another operational amplifier being supplied with an output of the operational amplifier through the third resistor, a non-inverting input of the another operational amplifier being supplied with the reference voltage; and a fourth resistor disposed between an output terminal and the inverting input of the another operational amplifier.